There are many situations where it is desirable to project high intensity beams of light for purposes of illumination. For example, automobile headlamps, flood lighting, vehicle reverse lamps, search lamps and the like. In certain cases it is required that the high intensity beams be projected at a high angle from the emission surface. For example trailer sidewall illumination lamps, marine docking lamps and the like.
In the past these high angle lighting devices have been created using incandescent light sources mounted with parabolic or elliptical reflectors to direct a beam of light in the required direction. This type of construction necessitates a certain amount of thickness to the lamp body. In many applications it is undesirable to have this thickness project from the mounting surface, and it is preferred to have a minimum amount of external projection from the surface. In order to mount this lamp to a surface, large holes must be cut in the mounting surface to allow the projection of the beam from the lamp housing.
For example, in the case of a marine docking lamp, a powerful light source is mounted in a plastic body and is typically angled at 60 degrees to the surface normal. The lamp source is typically 35 to 100 watts and has an integrated parabolic reflector which focuses the light into a powerful high intensity beam which is directed out of the housing through a transparent plastic or glass window.
This type of construction has several disadvantages. The primary drawback of existing docking lamps is that the light source is typically several inches in diameter requiring a large hole to be cut in the boat hull to minimize projection from the surface of the boat. The hole is elliptical in shape and must be accurately cut in relation to the bow to insure the projection angle from the boat hull is within defined guidelines. Boat hulls can be constructed from a variety of materials including fiberglass, steel or aluminum. Cutting an accurately positioned elliptical hole in thick fiberglass or metal can be quite costly. In addition, the hull structure may be weakened by formation of the hole thereby creating a potential water ingress point.
A second major drawback is the heat generated by convention large incandescent light sources. High temperature plastics, metals and glass are generally used for the lamp construction to reduce operator exposure to high surface temperatures. Nevertheless, boat owners have been burned by brushing up against the housings.
Another major drawback of the large powerful sources is the high current draw which requires large gauge wiring, and additional alternator and battery power for a marine vessel to operate. Battery and alternator power are at a premium on marine vessels where the energy is required for critical systems.
In addition to the difficulties in forming the hole in the boat hull, another drawback of conventional docking lights is that once the large hole is cut in the boat hull the point angle of the lamp cannot be easily adjusted. As a result, it is not uncommon for a boat to have docking lights with misaligned beams.
It is also recognized that when docking a marine vessel, a high intensity forward beam is preferred for viewing any hazards ahead of the boat but as the boat enters its slip it is advantageous to have a smaller amount of side light to provide visibility of the dock itself. The preferred light distribution is therefore a high intensity beam directed at approximately 60 degrees from the surface normal surrounded by a rectangle of lower intensity light having a wide beam width. When the lamp is mounted on the surface of the hull, the angle of the hull and the position of the lamp are combined to direct the highest intensity portion of the beam directly ahead.
In contrast to incandescent light sources, Light Emitting Diodes (LEDs) are solid state electrical devices with high efficiencies and long lives. LEDs are generally impact resistant, use very little power and often have 100,000 hour life spans. These features make these devices preferable for use in safety lighting. The primary disadvantage of LED light sources however is their cost. If the efficiency of an optical device to distribute light from the LED into the required or regulated pattern is improved, fewer LEDs can be used resulting in more cost accessible interior illumination and safety lighting devices.
It is recognized that operation and lifespan of an LED device is a partly a function of the temperature of the LED chip. Thus, for LED-based lamps it is generally preferred to maintain the temperature of the diode chip within a controlled temperature range. In addition to reduced LED life, higher LED temperature can affect the output color and intensity of the LED. Thus, to maintain the LEDs at a low operating temperature, LED based lamps are generally designed to have heat sinking and heat radiating features.
Recently, LED manufacturers have turned to surface mountable LED devices that have superior heat removal from the diode junction and higher optical flux per watt. These devices are now being regularly provided with a flat output surface free from the source distorting optics of previous LEDs. These devices typically have very wide output distributions with typical viewing angles greater than 100 degrees. The viewing angle is typically defined as the full angular width of the optical distribution where the light output reaches 50% of the intensity measured on the optical axis. LEDs of this type have generally symmetrical outputs around the center or optical axis. Thus, a device having a viewing angle of 10 degrees describes a conical output distribution where 50% of the peak intensity value occurs at 5 degrees from the optical or center axis of the device. A 120 degree viewing angle device, which is a very common wide output angle LED, defines a device which has an output intensity of 50% at an angle of 60 degrees from the optical axis. These LEDs have output intensity distributions which closely follow a Lambertian plane source emitter and emit light in a 180 degree hemisphere.
Notwithstanding the advantages of such surface mounted LEDs, they are not ideal for use as marine docking light lamps. The wide output of such powerful LED sources requires secondary lensing to collect and direct the light into a useful pattern. In order to efficiently meet light output requirements for a marine docking light application using a hemispherical emitting LED, the energy must be collected, concentrated and directed with the main part of the beam at an angle approximately 60 degrees from the surface normal.
It is generally accepted that when redirecting electromagnetic energy greater than 30 degrees from its emission direction it is advantageous to use reflection to change the light direction. Reflective surfaces can be created using metallization, dielectric coatings or by total internal reflection inside a transparent material. In production, dielectric coatings are often too expensive and are difficult to create on a curved surface. Metallic coating type reflectors typically have light absorption levels of 20% or more thereby reducing system efficiency. This makes it more desirable to use internal reflection whenever possible.
Internal reflection occurs when electromagnetic energy traveling through a transparent material strikes an outer surface at an angle to the surface normal greater than the critical angle for the material. One hundred percent of the light energy is reflected back into the lens material on a path according to the laws of reflection.
In many cases, internal reflection results in thick cross-section lens material and long beam paths inside the material. Thick materials are inherently difficult to mold as most materials shrink when cooling which can create internal stresses and surface deformations. Also, these thick materials often have long beam paths resulting in a need for high clarity materials to minimize beam attenuation.